Experience has shown that the catastrophic failure of an integrated circuit chip, such as a microprocessor, will typically occur during the initial phase of the chip life. If the chip passes its initial operational phase, the life and reliability of the chip will have a relatively high probability.
To precipitate early chip failure, the chip is "exercised" or powered while being subjected to relatively high external temperatures. Typically, a batch of chips is electrically powered in an oven where the temperature is maintained at approximately 150.degree. C. for an extended period of time, such as 1,000 hours. This is referred to in the art as "burn-in".
During burn-in, a batch of chips may be mounted on a mother board, and the chip leads are electrically connected to respective circuit elements on the mother board by a suitable means, such as one or more flexible electrical connectors. Maintaining good electrical contact between the chip leads and the flexible electrical connector is very important. Where the leads are made of gold, the electrical contact is usually not a major problem; but where the chip leads are tin plated, a relatively high retention force is necessary to assure good electrical contact. This is due to oxidation of the tin-plated chip leads. Even if the oven has a nitrogen atmosphere, it is still necessary to maintain a relatively-high clamping force to retain the chip frames and assure good electrical contact with the chip leads.
In order to assure a proper and sufficient retention force, especially where the chip leads are tin-plated, a system of levers or oversized latches are usually necessary in order to achieve the necessary mechanical advantage for the desired retention force. However, because of space problems, this is not feasible nor particularly desirable since it reduces the total number of chips on the board and thus reduces production rates.
For large batches of chips, the testing apparatus is fully automated and includes robotics arms for transferring the batches of chips into and out of the oven. In production, and because of the relatively large number of chips being burned-in, the prior clamping mechanisms interfered with the high-speed automated apparatus for transferring large batches of chips.
The co-pending and related application noted earlier represents an approach to overcome the problems associated with prior art procedures, particularly the clamping mechanisms. Such application teaches the use of a thermally-reactive resilient member, which as known in the art as a shape memory alloy (SMA). Briefly, such an alloy undergoes a dimensional change, through a crystalline or phase transformation, when subjected to a temperature increase. As part of the clamping mechanism, the SMA resilient means provides a relatively low force at ambient temperatures and a relatively high force at high temperatures, such that the clamping mechanism retains the chip frame, while maintaining good electrical contact between the electrical means and the chip leads during burn-in.
While the system of the co-pending application represents an effective manner to successfully test "chips", it does not have the full versatility of the present invention which avoids the use of SMA's, and is fully amenable to robotics handling. These and other features will become apparent in the description which follows, particularly when read in conjunction with the accompanying drawings.